1. Field of the Invention
This invention relates to laser systems. More specifically, it relates to improved laser systems that provide automatic correction of tilt and focus errors in laser beams.
While the present invention is described herein with reference to specific embodiments, it should be understood that the invention is not limited thereto. The principles of the present invention may be adapted and employed for a variety of requirements as those skilled in the art will recognize in light of the present disclosure.
2. Description of the Prior Art
Laser systems enjoy a wide variety of applications and promise a suitability for many others. Many applications require precise control of the direction and wavefront profile of the main laser beam. A wavefront is a three-dimensional surface of constant optical path length, orthogonal to a family of rays. Typical aberarations in the wavefront profile include those that change the focus, phase or astigmatic characteristics of the beam. Control of these distortions as well as the line-of-sight (tilt) of the main beam may be required in applications involving long distance communication and guidance of weapons systems. This is problematic insofar as the telescope invariably experiences microvibrations induced by the ambient environment. Such vibrations can seriously impair the performance of system.
In addition, the laser beam may be undesireably affected by distortions in the primary mirror or irregularities in the optical train between the laser source and the telescope. There are currently at least two approaches to this problem. One is to use four collimated pilot or auto-alignment beams in parallel with the primary beam. With this approach, the optical train is adjusted so that when it is properly aligned the pilot beams follow the optical train through the primary and secondary mirrors of the telescope and impinge upon several sensors gyro-stabilized along a vector in the line-of-sight (LOS) of the radar beam. The sensors provide signals to a servo-system which effectively corrects the aberration.
Such systems are costly and cumbersome in that additional laser beams, sensors, and optical components must be provided and each must be properly aligned.
Another approach utilizes a stabilized ring alignment concept. Here an annular reference mirror is disposed outside the primary mirror of the telescope so as to circumscribe the main laser beam. An auto-alignment beam parallel with the main beam, is reflected by the mirrored ring to a suitably located sensor. Here again, the sensor provides corrective signals to a servo-subsystem to compensate for the error. The auto-alignment ring can be quite large, and the fact that it must be inertially stabilized to high accuracies poses a limitation on the feasibility of this approach.
More importantly, while both the four beam autoalignment systems and the stabilized ring alignment systems provide a line-of-sight error correction capability, neither system can effectively detect wavefront aberrations arising from highly localized microdistortions in the optical train including the reflective surface of the primary mirror. The annular ring technique typically samples 1-10 percent of the aperture of the beam. The four beam auto-alignment systems typically sample 0.5 percent of the full aperture. Thus, the auto-alignment beams illuminate only a small portion of the reflective surface of the primary mirror.
Thus, it is a general object of the present invention to provide an improved automatic laser beam alignment system. It is a more specific object of the present invention to provide an improved laser beam alignment system that corrects line-of-sight and wavefront errors. An even more specific object of the present invention is to provide an improved automatic laser beam alignment system that is capable of correcting both distortions in the optical train and wavefront aberrations caused by microdistortions in the full aperture of the primary mirror. A still further object of the present invention is to achieve the above-identified objects with a small, simplified, economical error correction package that need not be inertially stabilized. These and other objects will become apparent upon consideration of the description which follows.